IPTV or PPV (Pay Per View) services are a business model that allows a viewer to purchase and view paid contents. These services generally adopt DRM (Digital Right Management) including a CAS (Conditional Access System) so that the services can be available only a viewer who made due payment. A CAS, as a technology for protecting an illegal user from accessing contents, allows only qualified users to view a specific program. Data can be scrambled by a transmitter, and the scrambled data can be de-scrambled by the permitted users, so that the user can view the program. FIG. 1 illustrates a general CAS-adopted system. Referring to FIG. 1, the CAS is generally implemented by a set-top box 11 that is provided by a service provider 10. Of course, a TV may include a set-top box function.
However, such CAS fails to prevent an authorized user to illegally view and copy contents. In general, images from the set-top box 11 are directed to an input terminal of a TV. As shown in FIG. 2, for connection between the set-top box and the input terminal of the TV, a device 15, such as a capture board, is connected to an output terminal of the set-top box 11, so that copied images with the same quality as the original images may be generated. Also, images displayed on the TV may be captured by a camcorder 16 to generate copied images. Such pirated contents are distributed to others through DVDs or an Internet file sharing service 19, thus causing huge damage to contents or service providers.
Such content piracy through an analog interface is referred to as an “analog-hole.” Various forensic watermarking technologies are suggested to address the illegal reproduction. Upon forensic watermarking, information relating to manufacturers or distribution channels or user's information are unnoticeably inserted into contents, so that when the contents are illegally distributed, information necessary to identify legality of the contents or to trace the distribution channels or distributers can be obtained.